1. Field of Endeavor
The present invention relates to a reheat burner.
2. Brief Description of the Related Art
Sequential combustion gas turbines are known to include a first burner, in which a fuel is injected into a compressed air stream to be combusted generating flue gases that are partially expanded in a high pressure turbine.
The flue gases coming from the high pressure turbine are then fed into a reheat burner, in which a further fuel is injected into the reheat burner to be mixed and combusted in a combustion chamber downstream of it; the flue gases generated are then expanded in a low pressure turbine.
FIGS. 1-3 show a typical example of a traditional reheat burner.
With reference to FIGS. 1-3, traditional burners 1 have a quadrangular channel 2 with a lance 3 housed therein.
The lance 3 has nozzles from which a fuel (either oil, i.e., liquid fuel, or a gaseous fuel) is injected; as shown in FIG. 1, the fuel in injected over a plane known as injection plane 4.
The channel zone upstream of the injection plane 4 (in the direction of the hot gases G) is the vortex generation zone 6; in this zone, vortex generators 7 are housed, projecting from each of the channel walls, to induce vortices and turbulence into the hot gases G.
The channel zone downstream of the injection plane 4 (in the hot gas direction G) is the mixing zone 9; typically this zone has plane, diverging side walls, to define a diffuser.
As shown in the figures, the side walls 10 of the channel 2 may converge or diverge to define a variable burner width w (measured at mid height), whereas the top and bottom walls 11 of the channel 2 are parallel to each other, to define a constant burner height h.
The structure of the burners 1 is optimized in order to achieve the best compromise of hot gas speed and vortices and turbulence within the channel 2 at the design temperature.
In fact, a high hot gas speed through the burner channel 2 reduces NOx emissions (since the residence time of the burning fuel in the combustion chamber 12 downstream of the burner 1 is reduced), increases the flashback margin (since it reduces the residence time of the fuel within the burner 1 and thus it makes it more difficult for the fuel to achieve auto ignition) and reduces the water consumption in oil operation (water is mixed to oil to prevent flashback). In contrast, high hot gas speed increases the CO emissions (since the residence time in the combustion chamber 12 downstream of the burner 1 is low) and pressure drop (i.e., efficiency and achievable power).
In addition, high vortex strength and turbulence level reduce the NOx and CO emissions (thanks to the good mixing), but increase the pressure drop (thus they reduce efficiency and achievable power).
In order to increase the gas turbine efficiency and performances, the temperature of the hot gases at the inlet and exit of the reheat burner 1 should be increased.
Such an increase causes the delicate equilibrium among all the parameters to be missed, such that a reheat burner operating with hot gases having a higher temperature than the design temperature may have flashback, NOx, CO emissions, water consumption and pressure drop problems.